Memory imaging is the process of making a bit-by-bit copy of memory. In principle it is similar to Disk Imaging.
For physical memory it is common to have sections that are not accessible, e.g. because of memory-mapped I/O
The resulting copy is stored in a Forensics image format. Some of these formats have means to differentiate between an image of memory and e.g. that of a disk.
Reading from the Physical Memory Object
In Windows the Physical Memory Object, \\Device\PhysicalMemory, can be used the access physical memory. Since Windows 2003 SP1 user-mode access to this device-object is no longer permitted . A kernel-space process is still allowed to read from this device-object.